The fresh-faced young Leader of the Opposition put away his papers and spoke without notes. The environment, he told us, was "at the heart" of his party's agenda and "absolutely essential to what we are about". He promised to follow up his speech "with commitment in opposition" and to "deliver on it in government".

I have to admit I was convinced by Tony Blair's words a decade ago and said so. Wiser heads - including Peter Melchett, then executive director of Greenpeace - urged cautious scepticism.

They were right. For the past eight years Downing Street has frustrated green policies and championed dodgy technologies from GM to nuclear power. And though the Prime Minister can justly claim to have led the world in drawing attention to the dangers of global warming - he has taken little action at home.

So David Cameron's enthusiastic espousal of green causes since becoming leader of the Conservative Party - also promising to put the environment "at the heart" of his policies - makes me a little wary.

Although he made green issues central to his leadership campaign - unlike David Davis who virtually ignored them - there is little evidence of previous interest. One policy aide told me that I would find it in columns he had written for a national newspaper website. Yet not one of 72 pieces written over more than four years was devoted to them.

Steve Hilton, one of the new leader's inner circle, says he has "always felt very passionate" about green issues, adding that his cycling around London has alerted him to air pollution while his hobby of growing vegetables has established a bond with nature. But it is still strange that he made nothing of them earlier.

Yet there are signs that he does, indeed, mean business. While the Tories still have few green policies, he can argue that these await the work of a policy commission which he set up, with the radical environmentalist Zac Goldsmith as deputy chairman, within days of becoming leader. And the one set of policies he has outlined, on global warming, are the most far-reaching proposed by any top Western politician.

Meanwhile he focused on climate change in his acceptance speech and at his first Prime Minister's Questions, and will make a major speech supporting organic farming on Thursday. He wrote to Tory supporters urging them to recycle Christmas cards, paper and bottles - and is even putting solar panels and a windmill on his new Notting Hill house.

As he is acutely aware, having put environmental issues to the fore, he cannot disappoint without making it seem that he is as little to be trusted as Tony Blair.

In Peter Ainsworth as environment spokesman and John Gummer as chairman of his commission, he has appointed people with real green credentials. Neither would have taken the jobs unless they believed he was serious.

Perhaps most important, he has made it clear that he will not allow business to determine his position and appears to understand that the environment is central to economic and social policies, and not a marginal issue.

The crunch will come this summer over the Government's bid to support a new generation of nuclear reactors. But, for now, environmentalists are impressed by the new Tory leader. The present head of Greenpeace, Stephen Tindale, says he should be greeted with "optimism rather than scepticism".

Maybe I've just got a trusting nature. Maybe I am being suckered for a second time. But so far it looks as if David Cameron is ushering in the most important breakthrough yet in the greening of British politics.